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CHEYENNE, Wyo. (AP) — Yellowstone National Park was jostled by a host of small earthquakes for a third straight day Monday, and scientists watched closely to see whether the more than 250 tremors were a sign of something bigger to come. Swarms of small earthquakes happen frequently in Yellowstone, but it's very unusual for so many earthquakes to happen over several days, said Robert Smith, a professor of geophysics at the University of Utah.

"They're certainly not normal," Smith said. "We haven't had earthquakes in this energy or extent in many years."

Smith directs the Yellowstone Seismic Network, which operates seismic stations around the park. He said the quakes have ranged in strength from barely detectable to one of magnitude 3.8 that happened Saturday. A magnitude 4 quake is capable of producing moderate damage.

"This is an active volcanic and tectonic area, and these are the kinds of things we have to pay attention to," Smith said. "We might be seeing something precursory.

"Could it develop into a bigger fault or something related to hydrothermal activity? We don't know. That's what we're there to do, to monitor it for public safety."

The strongest of dozens of tremors Monday was a magnitude 3.3 quake shortly after noon. All the quakes were centered beneath the northwest end of Yellowstone Lake.

A park ranger based at the north end of the lake reported feeling nine quakes over a 24-hour period over the weekend, according to park spokeswoman Stacy Vallie. No damage was reported.

"There doesn't seem to be anything to be alarmed about," Vallie said.

Smith said it's difficult to say what might be causing the tremors. He pointed out that Yellowstone is the caldera of a volcano that last erupted 70,000 years ago.

He said Yellowstone remains very geologically active — and its famous geysers and hot springs are a reminder that a pool of magma still exists five to 10 miles underground.

"That's just the surface manifestation of the enormous amount of heat that's being released through the system," he said.

Yellowstone has had significant earthquakes as well as minor ones in recent decades. In 1959, a magnitude 7.5 quake near Hebgen Lake just west of the park triggered a landslide that killed 28 people.

...scientists wonder when Yellowstone will blow again. According to LiveScience, the caldera last blew up about 600,000 years ago.

Projections suggest that such an eruption would be catastrophic to most of the United States, with half the country being “covered in ash up to 3 feet deep,” LiveScience says, adding, “But those same researchers say nothing suggests such an eruption is imminent. They point out, however, that Yellowstone seems to blow its top about every 600,000 years.”

The Cascades Volcano Observatory calls the Yellowstone caldera “one of the largest and most active in the world.” The United States has other, smaller calderas throughout the west.